Adult Activities

Outdoors

A Quick Guide to Plant GallsBy Education Officer Stephen Le Quesne

‘What is that? And why is there a lump on the bottom of that leaf?’ These questions were asked of me by a child with a keen eye whilst taking part in one of my Woodland Wander walks last year. I have to confess that I was not sure of the exact answer and so made it my business the next day to find out!

My findings were very interesting - plant galls are abnormal growths of plant tissues that are caused by various parasites from fungi and bacteria, insects and mites. The galls act both as a place of refuge and food source for the tiny young that are born inside. Usually, they are created by the injection of a special mix of chemicals.

Following this chance encounter I have become a little obsessed with these wonderful pieces of insect architecture. After much deliberation, research and exploring in the undergrowth, here are the galls that I highly recommend you go out and try to find.

Gall Name: Bramble GallLatin Name: Diastrophus rubi Grows on: Bramble stemsDescription: A rather large gall that can be found on bramble stems. The swellings are up to 15cm long, with a bumpy, uneven surface. Over 200 larvae may overwinter in the gall before they emerge in the spring, hence the numerous exit holes covering this strange growth.

Gall Name: Common Spangle GallLatin Name: Neuroterus quercusbaccarumGrows on: Underside of oak leavesDescription: This is a disc-shaped, hairy gall, up to 5mm across. It is very common on the underside of oak leaves during the autumn months. There is a slight rise in the centre, which is where the single gall wasp develops. The galls mature on the ground in the winter where it swells as the wasp inside develops. Come spring the insects leave to lay their eggs in oak buds.

Gall Name: OakMarble GallLatin Name: Andricus kollariGrows on: Turkey Oak budsDescription: This is one of our most common galls that starts off as a green colour in the summer but by the autumn is a dark, woody brown. Up to 20mm in size it is home to a single gall wasp, which leaves in the autumn. A small hole in the gall means that the wasp has already left.

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Gall Name: Knopper GallLatin Name: Andricus quercuscalicis Grows on: AcornsDescription: A gall that cannot be mistaken for any other gall, as it looks like a strange mouth trying to consume the acorn it has grown from. It is a soft gall that is up to 45mm across and actually contains numerous male and female gall wasps. Galled acorns fall from the trees in late summer with the adult insects emerging the following spring, although some may remain in their galls for up to four years!

Gall Name: Silk Button Spangle GallLatin Name: Neuroterus numismalisGrows on: Underside of oak leavesDescription: This gall is only 5mm across and, as with many others, grows only on the underside of oak leaves - and hundreds may occur on a single leaf! Here the galls fall from the leaves in the autumn and can sometimes be found on the woodland floor. The small wasps stay inside the gall buried underground until the spring when they emerge.

Gall Name: Pea GallLatin Name: Cynips divisaGrows on: Underside of oak leaves Description: Also known as the red currant gall, it grows on the veins of the leaves with up to fifteen of them occurring on a single leaf. They are slightly flattened and very hard. These galls start off as green when they appear in the summer, but quickly change to yellow, orange and then red-brown by the autumn. The galls fall in the autumn and the adults emerge in the winter to lay their eggs.